By early Saturday afternoon, there were so many corpses arriving at Cairo's Zeinhom mortuary that the street outside was blocked with a queue of orange ambulances. Inside one of them, mechanical engineer Mohamed Khamis waited with the body of his 15-year-old son, Omar, shot in the head by police.

"He will go back to school this autumn, God willing," said Mohamed, struggling to come to terms with Omar's death, his hands still covered in his son's dried blood.

Six hours earlier, both father and son had been surveying the scene of Cairo's most recent massacre. They had taken care to avoid the frontline, but suddenly they heard gunfire close by. Mohamed turned to run.

"And as I turned, I felt him fall on my shoulder," said Mohamed, his body shaking slightly. "I put my hand out to catch him and his head fell on my hand. I felt his crushed skull. There was blood on the floor. He was already dead."

Omar Khamis was one of at least 100 pro-Mohamed Morsi supporters killed by state officials in an eight-hour-long massacre on Saturday morning – Egypt's second mass killing of Islamists in three weeks. In post-revolutionary Cairo, now more divided than ever after the toppling of Morsi on 3 July, the narrative of history is rarely straightforward. On Saturday the city was awash with claims and counterclaims about whether the bloody events had been provoked.

According to Egypt's interior ministry, pro-Morsi supporters, who have camped in their tens of thousands outside the Rabaa al-Adawiya mosque in east Cairo since Morsi's removal, tried to extend their camp at around midnight on Friday as far as the nearby memorial for another of the country's fallen presidents, Anwar Sadat, who was assassinated in 1981. Officials said the protesters had fired live ammunition at police when they tried to clear the alleged new campsite, which forced the police to respond in kind.

But the protesters have a different story – that two separate pro-Morsi marches returning to Rabaa al-Adawiya after circling the surrounding area found the site so crammed that they could not re-enter it. Many were forced instead to sit down outside Sadat's memorial, several hundred metres up the road.

Then officers and armed men dressed in civilian clothing started to fire on them from a flyover beyond the memorial – first, with teargas and shotgun pellets.

Fearing that if they left their position the police would seek to take the camp itself, Morsi's supporters – many of them from the Muslim Brotherhood – responded with rocks, fireworks and spent teargas canisters. The fighting lasted from the small hours of the morning until 8am or 9am. No state officials were reported to have been killed.

"There must have been an injury every minute," said Mosa'ab Elshamy, a photojournalist unaffiliated with the Morsi movement, who photographed the Islamists' frontline for half an hour at around 4am.

"I did not see any Morsi supporters with [firearms] at this point," said Elshamy, while refusing to rule out the possibility that some may have been firing live ammunition. "I hid behind a tree, and all I saw were Morsi supporters throwing stones, or fireworks, or throwing teargas canisters … They just wanted to hold their ground. They were protecting the sit-in because they believed that, if they left, the police would follow them."

At 7am a medic treating the wounded at the site said he saw police shooters target those rescuing the wounded. "Even at that time, people were still dropping like flies," said Dr Ahmed Said, a volunteer at the Rabaa al-Adawiya field hospital, a clinic set up originally to deal with minor injuries.

Back at the field hospital itself, medics could not cope with the number of bodies being brought back. "By 7am or 8am, doctors were falling down with exhaustion," said Dr Alaa Mohamed Abu Zeid, a radiologist volunteering at the hospital. All were observing Ramadan, so many had not had time to eat or sleep before they were called into action.

"Nobody should see what we had to see today," said Amr Gamal, a young doctor at the clinic. "It was like a war zone. The whole area was so full with bodies that we couldn't move."

By the time the Observer arrived in the late morning, the hospital was in chaos – many bodies already evacuated, but the floors smeared with blood, and strewn with used and bloodied surgical gloves. One man held the bloodstained wallets and mobile phones of the dead. Another held a stack of their ID cards. In the next room 16 bodies lay scattered on the floor, doctors and family members clambering over them – some screaming. Several people prayed, while others chanted: "The people demand the execution of Sisi" – a reference to Abdel Fatah al-Sisi, the army chief who forced Morsi from power, and who earlier in the week called for Egyptians to back his campaign against what he termed terrorism. Cynics saw Sisi's speech as a veiled reference to a brutal crackdown on Morsi's Muslim Brotherhood – a prophecy that came true on Saturday.

Some casualties reported seeing police or army snipers firing on protesters from buildings inside the nearby Al-Azhar University, and medics said the accuracy of the shooting suggested that snipers may have been in action.

"The injuries were very precise – which suggests they were shot by snipers," said Dr Mohamed Lotfy, in charge of the clinic's medical supplies. "There were bullet holes in the centre of the forehead and right in the back of the skull. It was not just shooting to injure. They were shooting to kill."